US3922523A

US3922523A - Apparatus for producing x-ray images as radiographs

Info

Publication number: US3922523A
Application number: US360753A
Authority: US
Inventors: Edward Emanuel Sheldon
Original assignee: Sheldon Edward E
Current assignee: UNITED JEWISH APPEAL OF GREATER NEW YORK
Priority date: 1973-05-16
Filing date: 1973-05-16
Publication date: 1975-11-25
Anticipated expiration: 1992-11-25

Abstract

This invention relates to the novel system of radiography. The image of examined object is formed by X-radiation. This image is converted in a vacuum tube into a beam of electrons. The beam of electrons is converted next into an electrical pattern and is stored on a movable sheet of dielectric material which travels through said vacuum tube. Means are provided for feeding said sheet or tape of dielectric material into said vacuum tube and for transporting it after X-ray exposure to the outside of said vacuum tube. The stored image is next developed, is made visible and is fixed with or without transfer to another medium in order to produce the final visible radiograph for inspection.

Description

United States Patent Sheldon Nov. 25, 1975 [54] APPARATUS FOR PRODUCING X-RAY 3,453,639 7/1969 Berman 1. 346/110 V IMAGES AS RADIOGRAPHS 3,515,870 6/1970 Marquis 1 1 1 u 250/320 2,877,368 3/1959 Sheldon 313/ [76] Inventor: Edward Emanuel Sheldon, 30 E. 40

New York NY 0016 Primary Examiner-1ames W. Lawrence [22] Filed: May 16, 1973 Assistant Examiner-B. C. Anderson [21] Appli No.: 360,753

ABSTRACT [52] US. Cl. 250/320; 250/483; 250/475 This invention relates to the novel system of radiogra- [51] Int. Cl. G0ln 23/04 phy. The image of examined object is formed by X- [58] Field of Search 250/315, 320, 321, 323, radiation. This image is converted in a vacuum tube 250/483, 475; 346/110 V into a beam of electrons, The beam of electrons is converted next into an electrical pattern and is stored [56] References Cited on a movable sheet of dielectric material which travels UN|TED STATES PATENTS through said vacuum tube Means are provided for 2l98479 4/1940 Langmuir H 250/320 feeding said sheet or tape of dielec tric material into 2,221,776 H1940 Carlson V p I v I I I 4 H /5 said vacuum tube and for transporting it after X-ray 2,440,640 4/1943 Manon 250/320 .exposure to the outside of said vacuum tube. The 155 423 /195 Sheldon 29 71 stored image is next developed, is made visible and is 3,021.834 2/1962 Sheldon 128/6 fixed with or without transfer to another medium in 3,099,710 7/1963 M611er et a1. 346/ V order to produce the final visible radiograph for in- 3,236,943 2/1966 Mdller 346/110 v spec/ion 3,281,858 10/1966 Schwertz 346/110 V 3,400,291 9/1968 Sheldon 1. 313/65 9 Claims, 6 Drawing Figures C'AMAG/NG CAM/ I 7 /7 .1- L, 1 5 6 mi l 1 1 /6 WV I 8 /5 r VACUUM LOCK 17 0E VELOP/NG S R 0; 04444552 20 P075V774L l4 fi7X/A/6' cmswaee 2/ IPEL-L APPARATUS FOR PRODUCING X-RAY IMAGES AS RADIOGRAPHS This invention relates to a novel system of radiography which means to the apparatus for producing pictures formed by X-radiation. The present systems are all based on the use of films made with silver halides and which are used in combination with Xray sensitive fluorescent screens. The present radiographic systems do not provide any means for intensifiction of the X-ray image and as a result a large amount of X-radiation is necessary to produce a radiograph. This may be not of crucial importance in industrial radiography but is of great importance in the medical radiography in which the X-ray exposure of patients should be kept at minimum. In view of the fact that the number of X-ray examinations increases greatly every year, the concern is voiced that many patients, especially of reproductive age receive too large amount of X-radiation which may affect their genetic system. This means that the number of congenital malformations in new-born may increase greatly. It is therefore of great importance to provide a system of radiography which will reduce the X-ray exposure of patients and still will provide the necessary diagnostic information. This is the main objective of the present invention.

Another objective of this invention is to reduce the costs of X-ray examination. In view of multiple and extensive X-ray examinations which are necessary in hospital work, the cost of X-ray examinations are very high. The present invention, by eliminating the use of expensive X-ray film and substituting them with very cheap plastic materials will greatly contribute to the reduction of X-ray examination expenses.

Another objective of this invention is to produce X-ray images of better diagnostic quality than it was possible using the present methods of radiography.

The above objectives were realized by the novel system of radiography which is characterized by the use of electronic amplification of X-ray images in a novel vacuum tube and of inexpensive materials such plastics for reproducing X-ray images in a visible form as radiographs.

The invention will be better understood when taken together with the accompanying drawings.

IN THE DRAWINGS FIG. 1 shows the new apparatus for radiography which comprises an X-ray sensitive vacuum tube and dielectric film travelling through said tube.

FIGS. l-A and 1-]! show modification of the X-ray sensitive screen used in the vacuum tube illustrated in FIG. I.

FIG. 2 shows a modification of the apparatus for radiography in which the X-ray sensitive vacuum tube contains a layer of material which exhibits electron bombardment induced conductivity, said layer movable through said tube.

FIG. Z-A shows a modification of the layer used in the embodiment of FIG. 2.

FIG. 3 shows a modification of the apparatus for radiography in which a film comprising a layer of photoconductive material is used in a vacuum tube.

FIG. 1 shows the X-ray source 15, the examined object I6 and the novel X-ray sensitive image tube 1 which comprises an evacuated envelope 2. The X-ray sensitive photocathode or screen 3 may be mounted 2 on the inside surface of the endwall of said tube or it may be mounted in a spaced apart relationship from said endwall. In the latter case it will be supported by the supporting member 5 such as of aluminum or beryllium. The photo-cathode 3 comprises furthermore fluo rescent layer 6 of one of phosphors such as of CsI or ZnSCdS or CaWO, or of mixture thereof and of photoemissive layer 7 such as of CsSB or K-Cs-Sb, Na-KSb or other photo-electric materials of multi-alkali type. The X-ray image is converted in said photo-cathode 3 into a fluorescent image and next into a beam of free photo-electrons corresponding to the X-ray image. The beam of photo-electrons is accelerated to the necessary velocity by electrostatic electrodes and then is focused by electrostatic or magnetic focusing means 4 onto electron reactive dielectric film 8. In some cases as shown in FIG. l-A the focusing electrodes may be omitted and photo-electron or electron beam may be focused onto film 8 by proximity focusing. In such case the spacing between the photo cathode 3b and film 8 should not exceed 2mm and the photo-cathode 3h will have a planar shape. The electron reactive film 8 may be one of flexible or semi-flexible plastics of a high electrical resistivity such as polyesters, polyamides,

polyethylenes or polycarbonates. Preferably a transparent material should be used but this is not obligatory.

In another embodiment of invention the film 8 comprises a continuous or mosaic layer of material which is a good emitter of secondary electrons such as MgO or alkali halide which is mounted on the surface of film 8 which faces the photo-cathode 3 and receives the photo-electron beam. The impingement of said photoelectron beam will produce secondary electron emission which may be collected by a suitable collector electode and will leave positive or negative electrical charge on the surface of said electron emitting layer. The film 8 is mounted to be able to travel into the vacuum tube 1 from the film supply chamber 22 which comprises film feeding mechanism. After the exposure by the photo-electron beam in said vacuum tube film 8 is transported outside of said tube 1 to developing chamber 20. Vacuum safety locks 17 serve for the entry and exit of film 8.

The film 8 may be of planar shape or may have curvature at the time of the exposure to the electron beam. The curvature of film 8 will depend on electronoptics used in vacuum tube I and may be produced by the use of supporting electrode 10 which is provided with the necessary curvature. The electrode 10 may be connected to the source of potential and provide the necessary voltage to produce secondary electron emission from the film 8 either higher than unity or lower than unity according to the application of this apparatus. The collector electrode for secondary electrons may be in the form of mesh screen 18. In the embodiment of invention which uses electrostatic focusing means the film 8 or its modifications may have a curvature with the concavity facing the concavity of the photo-emissive layer or it may be also of planar shape. In the embodiment in which magnetic focusing means are used and also in modification in which the proximity focusing is used, the film 8 will have planar shape parallel to the planar shape of the photo-emissive layer.

The dielectric film 8, in some applications receives in a charging chamber 19 before its introduction into the X-ray sensitive vacuum tube I an electrostatic uniform charge of positive sign on its surface which will face the photo-cathode 3. The dielectric film 8 positively precharged will be now introduced into vacuum tube 1 and will be bombarded by the photo-electron beam. In this modification the velocity of photo-electron beam must be such that it produces secondary electron emission of less than unity and forms thereby a negative charge image on film 8. This negative charge image is immediately neutralized by the positive electrostatic charge which was deposited on film 8 previously. As a result the remaining positive charge on film 8 will have the pattern of the original photo-electron beam and therefore of the original X-ray image. The film 8 is now transported out of the vacuum tube 1 and travels into developing chamber which may have various forms well known in the art and which may use a toner for said development. The film 8 with the developed image is transported now into transfer and fixation chamber 21 to produce the permanent image. If the transfer of image onto another material is desired, it can be done by well-known transfer techniques. The final image may he therefore in the form of transparency and can be examined with transmitted light like any conventional X-ray film, or it may be in non-transparent form in which case it will be examined with reflected light In another modification of invention the film 8 is charged uniformly with electrostatic charge of negative sign. In this embodiment the photo-electron beam is accelerated to velocity in which the secondary electron emission is higher than unity. The secondary electrons are led away by the mesh screen 18. As a result. the positive charge is formed on film 8 and it is neutralized by the negative electrostatic charge deposited previously. In this way the remaining negative charge will have the pattern of the original photo-electron beam which again had the pattern of the original X-ray image. The rest of developing and fixing and transfer pro cedures is the same as was described above.

It should be understood that in many applications film 8 may be used without pre-charging it with electrostatic charge as was described above. In such case the electrostatic charge will be applied to film 8 in the developing chamber 20.

It was found however that in some applications such as in medical radiography where the imaging X-radiation has to be very low in order not to harm patients, the above described devices do not have the sensitivity which is necessary. It was found that in examinations of heavy parts of patients such as abdomen, it is necessary to improve the sensitivity by a factor of 100-200. In order to obtain such improvement, it is necessary to increase the current density of the photo-electron beam without increasing the amount of X-radiation. This was accomplished by using cascade intensification in which another composite screen made of a light reflecting layer, a fluorescent layer, a light transparent supporting member and of a photo-emissive layer in that order is mounted in spaced relation to the photo-cathode 3. The photo-electron beam from photo-cathode 3 is accelerated to 15-20 KV before it impinges on the second composite screen producing thereby intensification of photo-electron beam by a factor of 50-100 as it was described in my U.S. Pat, No. 2,555,423. It may be added that the supporting light transparent layer for the second composite screen may be of vacuum resistant plastic material such as polyamide or silicone in order to be able to make this second composite screen of size necessary in medical radiography. In some cases the fluorescent layer may be mixed with a light transparent 4 separating layer to make a self-supporting member of this mixture.

This construction will give intensification by factor which will be still insufficient for Xray examinations of heavy parts. The sensitivity should be improved by an additional factor of 2 to 3. This was accomplished by coating the surface of dielectric film 8 which is exposed to the primary beam of photo-electrons or electrons from the X-ray reactive screen 3 with a layer of good secondary electron emitter such as MgO or an alkali halide. A good secondary electron emitter means material which emits more of secondary electrons than the dielectric film 8. For example, secondary electron emission of plastics is limited to 2 secondary electrons for one primary electron, whereas MgO will emit 4-5 secondary electrons for one primary electron. The layer of good electron emitter may be a continuous layer if the said material is insulator or should be ofmosaic type if said material is of semi-conducting or conducting type.

Another way to amplify the photo-electron beam is to use a multi-channel multiplier as described in my U.S. I-atv No. 3,400,291.

Another way to amplify the photo-electron beam is by the use of an electron multiplier oftransmission type such as a membrane of silicon preferably of p-type.

All these amplifying means will increase the sensitiv ity of this apparatus for radiography by a factor of IOU-2000. The above described amplification allows to use the very low sensitivity dielectric film 8 without increasing the amount of X-radiation and such combination represents an important feature of this invention.

In all embodiments of invention it is advantageous for better sensitivity to demagnify the photo-electron beam electron-optically. This is also of great importance as it permits to reduce the size of the amplifying stage which will allow to make the apparatus of a smaller size and of a lower cost.

In some applications it is preferable to provide ampli fication of the photo-electron beam by other means than described above which are technologically complicated. FIG. 2 shows a simplified and efficient solution to the problem of increasing sensitivity of vacuum tube 1 and of the system of Radiography. In this embodiment of invention the film 8 is replaced with the composite film 25 which comprises a layer of dielectric material 26 which when being bombarded by electrons or photo-electrons produces electrical conductivity changes. Such layer may be of MgO, alkali halides, ZnS or CdS. Next to layer 26 is layer of electrically conducting material 27 which may be of transparent type such as Nesa or indium oxide or of nontransparent material such as ofa metal like Al or Cu. Next to layer 27 is the supporting layer 28 which may be of any flexible material such as of plastics, e.g., polyesters, polyamides, or acetates. The layer 27 is connected to a suitable source of electrical potential 29 or to a ground. The rest of vacuum tube la will have a similar construction as the tube 1 shown in FIG. 1 or in l-A or in 1-D. The member 40 of electrically conducting material may be connected to the source of electrical potential or to the ground and may support film 25 or may be spaced apart from it. The photo-electron beam from the photo-cathode 3 is accelerated to 10-20 KV velocity and when it impinges on layer 26 it produces therein a pattern of electrical conductivity changes which corresponds to the original X-ray image. This pattern of electrical conductivity will persist in a suitable material for a long time. The film 25 will be now transported to the charging chamber 3] in which an electrostatic uniform charge will be deposited on the free surface of layer 26. This charge will leak away due to electrical conductivity induced in layer 26 and the remaining charge will have now the pattern of the original X-ray image. Next the film 25 is transported into developing chamber 32 in which a suitable toner is applied to make this charge visible. Next the film 25 is transported to chamber 33 in which it is transferred to another film and after the transfer it is fixed to produce a permanent image. The new transfer material may be preferably of transparent type such as of acetate, polyester, polycarbon or polyamide. The film 25 after cleaning and neutralization of charges will be recycled for the next exposure. In this way the new device provides an inexpensive system of radiography in which a cheap plastic film will replace expensive X-ray films.

FlG. 2-A shows another embodiment of the film 25 in which only 2 layers are present. The film 35 consists of layer 36 which is of dielectric material such as MgO. alkali halides, ZnS or CdS or Sb S and exhibits electron bombardment induced conductivity. Next is the layer 37 which is of flexible and electrically conducting material such as copper laminate or other metal laminate and which is made thick enough to be self-supporting. The layer 37 is connected to the source of electrical potential 29 or to ground. The planar or curved member 40 may be used to provide support for the film 25 or 35. It may also serve to provide the necessary curvature for the film 25 or 35 in some cases. It may be also connected to the source of electrical potential to provide high acceleration for the electron beam, such as -20 KV which is necessary to produce electrical conductivity by electron bombardment. The member 40 may be conversely connected to the ground, if the photo-cathode is held at high negative potential.

In some applications the film or 35 may be precharged with a uniform electrostatic charge before introducing said film into vacuum tube 1 or its modifications. ln such case its subsequent deposition of electri cal charge after removal of film 25 or 35 from vacuum tube may be omitted. This embodiment of invention is ofimportance if the materials used for film 25 or 35 exhibit induced electrical conductivity for a short time only. The electron bombardment induced conductivity can produce amplification of the electron beam by the factor of 100 to 1000 and allows therefore to eliminate lother amplification means described above, in some applications. In other cases however there may be need for additional amplification of the electron beam and in such cases the use may be made of all amplifying devices which were described above. In particular, it should be understood that in all devices described above the use may be made of amplification of the electron beam either by means of another stage composite screen, or of a multi-channel electron multiplier or of transmission type of electron multiplier such as silicon membrane of p-type. It should be understood that in all devices described above the focusing means for electron beam may be of electrostatic type or of magnetic type or of proximity focusing type.

It should be understood that in all embodiments of invention the final radiographic image may be made on a transparent substrate or on light reflecting substrate. It should be understood that in all embodiments of invention the final radiograph may be made of either polarity which means that black and white areas of radio graph may be reversed according to the need of application. It should be understood that in all embodiments of invention the final radiograph may be produced on the original film which was used in a vacuum tube 1 or its modifications or may be produced on another film which serves as a transfer medium.

In conclusion, an efficient and inexpensive new system of radiography is provided by the apparatus described in FIGS. 2 and Z-A and their modifications which will provide at the same time better X-ray examinations and at a much lower cost.

FIG. 3 shows another embodiment of invention which will be useful in some applications. In this em bodiment the vacuum tube 45 comprises X-ray reactive screen 46 of one of modifications described above and in addition a composite fluorescent screen 47. Screen 47 comprises light reflecting layer which is pervious to electrons, e.g., of aluminum 48, fluorescent layer 49 of one of phosphors described above and a light transpar ent supporting layer 50 of one of plastics such as polyesters or polyamides or silicones. The layer 50 may also be in the form of a fiberoptic member which is constituted of an array of light conducting fibers. Each of said light conducting fibers conducts light by internal reflection and comprises core of transparent material of a high index of refraction such as quartz or suitable plastics such as polycarbonates or acrylates. Each of said fibers has a coating of material of a lower index of refraction. The coating may be only of a few microns thickness. The plurality of such fibers assembled together forms an array which can conduct the image by internal reflection from one end of said array to the other end of said array, as it is described in my U.S. Pat. Nos. 2,877,368 and 3,021,834. The X-ray image is converted into an electron beam which represents the orig inal Xray image. This electron beam is demagnified by electron-optical means and is accelerated to high emergy such as 20 KV to excite the fluorescent layer 49 and to produce fluorescent image corresponding to X-ray image. The fluroescent image is conducted by the supporting member 50 and irradiates the image reproducing film 55. In some cases the electron beam is not demagnified but is of original size. This is the case when proximity focusing is used.

The image reproducing film 55 has different construction from the film 25 or 35 because the first layer which receives the fluorescent image is the layer 56 which is of dielectric material and which has also photo-conductive properties. Such materials are Se, Zns and CdS and Sb S with suitable activators such as As or Te. The next layer 57 is electrically conducting material and may be of light transparent type such as Nesa or indium oxide or of light non-transparent material such as a metal. The next layer 58 is a supporting member which is of flexible and dielectric material and which may be of light transparent type or of light impervious type. Materials such as plastics described above or fiber glass or laminates of glass may be used for layer 58. The film 55 may be also simplified to consist of two layers only. In this embodiment the electrically conducting layer 57 is made thick enough to serve as a supporting layer and is constructed to be flexible. It may be therefore made of laminates of metals such as copper or other metals which provides the necessary flexibility. The electrically conducting layer 57 may be connected to the source of electrical potential or to ground. The film 55 is mounted in a very close spacing to the supporting member 50 such as not exceeding lOO microns. In cases in which the supporting member 50 is made of fiberoptic plate described above, film 55 may be in contact with the member 50. The supporting member 50 may be of planar shape or of curved shape according to the electron-optical system used. In cases in which the supporting member 50 is of plastic the film 55 may be self-supporting or may rest on the support in the form of separate member 40 as was shown in FIGS. 1 and 2. The photoconductive layer 56 of film 55 when exposed to the fluorescent image from the layer 49 responds with changes of its electrical conductivity which have the pattern of said fluorescent image. This photo conductivity pattern will persist in a photo-conductor material of storage type for a long time. Materials such as Sb S with activators such as As and Te have very good storage properties. The film 55 may be now transported through vacuum safety locks 17 to the outside of vacuum tube 45 into charging chamber 31 in which it receives a uniform electrostatic charge as described above. The electrostatic charge leaks away through the areas which were made electrically conductive by fluorescent image. The remaining charge will be the replica of the original fluorescent image. The film 55 is now transported to the developing chamber 32 in which a toner in the form of one of black powders is applied to visualize the pattern of electrostatic charges. Next the film 55 is transported to the transfer chamber 33 where the electrostatic charge with toner is transferred to the final supporting sheet which may be oflight transparent or light impervious material and is fixed therein as was described above.

It should be understood in all devices described above, the X-ray reactive photocathode 3 may be of composite screen type described above and illustrated in FIGS. 1 or 1A; or may be of a metal 3a of high atomic number such as tungsten or lead as illustrated in FIG. 1B; or may be of a neutron-sensitive type as described in my U.S. Pat. No. 2,555,423.

It should be understood that in all devices described above the use may be made of amplification ofthe electron beam either by means of another stage composite screen, or of a multi-channel electron multiplier or of transmission type of electron multiplier such as silicon membrane of p-type. It should be understood that in all devices described above the focusing means for electron beam may be of electrostatic type or of magnetic type or of proximity focusing type. It should be understood that an ion pump is intended to be used in all embodiments of invention for the best gettering action.

It should be understood that dielectric material described above mans a material which has electrical resistivity not less than ohm-cm. It should be under stood that in all embodiments of invention said means for receiving electron beam may be informed of a sheet or tape and may be flexible, semi-flexible or non-flexible.

It should be understood that in all embodiments of invention the final radiographic image may be made on a transparent substrate or on light reflecting substrate. It should be understood that in all embodiments of invention the final radiograph may be made of either polarity, which means black and white areas of radio graph may be reversed according to the need of appli 8 cation. It should be understood that in all embodiments of invention the final radiograph may be produced on the original film which was used in a vacuum tube or may be produced on another medium which serves as a transfer medium.

It should be understood that the particular embodiments and forms of this invention have been illustrated and it is understood that modifications may be made by those skilled in the art without departing from the scope and spirit of the foregoing disclosure.

What I claim is:

1. An apparatus for medical radiography for X-ray examination of human body comprising in combination a vacuum tube comprising an X ray reactive screen producing in response to an X-ray image a beam of electrons corresponding to said image, image storage means receiving said beam of electrons and intensifying said beam, said image storage means comprising a layer of material exhibiting electron bombardment induced conductivity and an electrically conducting member, said image storage means being movable from the outside of said tube into said tube and movable from said tube to the outside of said tube, said vacuum tube comprising in addition vacuum safety locks for protecting vacuum of said tube during removal of said image storage means from said tube. and means for converting said stored and intensified image moved outside of said vacuum tube into a visible image for inspection.

2. A device as defined in claim 1 in which said electrically conducting member is connected to a source of electrical potential mounted outside of said vacuum tube.

3. A device as defined in claim I in which said image storage means are flexible.

4. A device as defined in claim 1 in which the whole of said stored and intensified image is converted simultaneously into a visible image.

5. A device as defined in claim 2 in which said image storage means are flexible.

6. A device as defined in claim 2 in which the whole of said stored and intensified image is converted simultaneously into a visible image.

7. A vacuum tube for medical radiography of human body comprising in combination an X-ray reactive screen producing in response to an X-ray image a beam of electrons corresponding to said image, image storage means receiving said beam of electrons, and intensifying said beam, said image storage means comprising a layer of material exhibiting electron bombardment induced conductivity and an electrically conducting member, said image storage means being movable from the outside of said tube into said tube and movable from said tube to the outside of said tube, said vacuum tube comprising furthermore vacuum safety iocks protecting the vacuum of said tube during removal of said image storage means from said tube.

8. A device as defined in claim 7 in which said electrically conducting member is connected to a source of electrical potential mounted outside of said vacuum tube.

9. A device as defined in claim 7 in which said image storage means are flexible.

* i l i

Claims

1. An apparatus for medical radiography for X-ray examination of human body comprising in combination a vacuum tube comprising an X-ray reactive screen producing in response to an X-ray image a beam of electrons corresponding to said image, image storage means receiving said beam of electrons and intensifying said beam, said image storage means comprising a layer of material exhibiting electron bombardment induced conductivity and an electrically conducting member, said image storage means being movable from the outside of said tube into said tube and movable from said tube to the outside of said tube, said vacuum tube comprising in addition vacuum safety locks for protecting vacuum of said tube during removal of said image storage means from said tube, and means for converting said stored and intensified image moved outside of said vacuum tube into a visible image for inspection.

3. A device as defined in claim 1 in wHich said image storage means are flexible.

7. A vacuum tube for medical radiography of human body comprising in combination an X-ray reactive screen producing in response to an X-ray image a beam of electrons corresponding to said image, image storage means receiving said beam of electrons, and intensifying said beam, said image storage means comprising a layer of material exhibiting electron bombardment induced conductivity and an electrically conducting member, said image storage means being movable from the outside of said tube into said tube and movable from said tube to the outside of said tube, said vacuum tube comprising furthermore vacuum safety locks protecting the vacuum of said tube during removal of said image storage means from said tube.